The present invention relates to an apparatus for conveying sheet materials so as to laminate two sheet materials to each other using a conveyor constituted by a hollow drum having holes disposed in a circumferential surface thereof, endless belts wound on the hollow drum so as to be driven to rotate in sliding contact with the hollow drum, and a suction device for sucking air out of the hollow drum.
As an image forming apparatus, there is a system in which an image is formed by laminating two sheet-like materials to each other. Examples of such a system include a system in which an exposed photosensitive material and an image reception material are laminated on each other so as to form an image on the image reception material, and a system in which an original and a photosensitive material is laminated on each other and the lamination is exposed to thereby form an image on the photosensitive material.
In such an image forming apparatus, when a sheet-like photosensitive material, an image-reception material, an original, and the like are conveyed to an image forming section, various kinds of conveyors are employed. For example, there are a roller-type conveyor in which nip rollers sandwich a sheet material therebetween and rotate to thereby convey the sheet material, and a belt-type conveyor in which a pair of belts sandwiching a sheet material therebetween are driven to thereby convey the sheet material. Further, as a modified example of the belt-type conveyor, there is a conveyor in which vacuum pressure is applied to a sheet material so as to closely adhere the sheet material to the belts to thereby convey the sheet material by the belts.
The last-mentioned conveyor is constituted by a hollow drum having holes disposed in a circumferential surface thereof, a plurality of endless belts wound on the hollow drum so as to be driven to rotate while being in sliding contact with the hollow drum, and a suction device for sucking air out of the hollow drum (hereinafter, this conveyor is referred to as "an air-suction-type conveyor"). The air is sucked out of the drum by the suction device so as to provide a vacuum inside the drum to thereby adhere the sheet material closely to the belts, and the belts are rotated to convey the sheet material.
In the case where sheet materials are conveyed by such a conveyor so that two sheet materials are laminated on each other, when the sheet materials are not conveyed in the proper direction, the sheet materials are respectively corrected by a correction device and then the two sheet materials are laminated on each other.
When a sheet material is conveyed by the air-suction-type conveyor various factors may cause variations in the rotational speed of the conveying belts so that the sheet material is curved in the conveying direction or biased left or right. Such factors include, for example, variations in the diameter of the shaft of the driving rollers or vibrations therein, variations in circumferential length and in thicknesses of the endless belts, or nonuniform load or variations in the circumference of the tension roller.
Further, sheet materials which tend to easily retain static electricity, or sheet materials having different rigidity characteristics are curved or biased in the left and right directions while the sheet materials are being conveyed. Specifically, in the image forming apparatus, if an original and a photosensitive material are laminated on each other without being corrected in the conveyance direction, accurate image formation is not performed. Therefore, not only is a proper image not obtained, but jamming of the sheet materials is caused in the image forming apparatus.
Further, the sheet materials are always closely adhered to the belts in the air-suction-type conveyor, so that even if the direction of conveyance of the sheet material is attempted to be corrected by a correction device, such attempt may not be successful so that the sheet materials may not be conveyed properly relative to the belts.
Accordingly, the direction of the sheet materials cannot always be corrected.